Re: [PATCH v1 0/3] introduce priority-based shutdown support

[Christian Loehle]

On 25/11/2023 08:50, Oleksij Rempel wrote:
> On Sat, Nov 25, 2023 at 06:51:55AM +0000, Greg Kroah-Hartman wrote:
>> On Fri, Nov 24, 2023 at 07:57:25PM +0100, Oleksij Rempel wrote:
>>> On Fri, Nov 24, 2023 at 05:26:30PM +0000, Greg Kroah-Hartman wrote:
>>>> On Fri, Nov 24, 2023 at 05:32:34PM +0100, Oleksij Rempel wrote:
>>>>> On Fri, Nov 24, 2023 at 03:56:19PM +0000, Greg Kroah-Hartman wrote:
>>>>>> On Fri, Nov 24, 2023 at 03:49:46PM +0000, Mark Brown wrote:
>>>>>>> On Fri, Nov 24, 2023 at 03:27:48PM +0000, Greg Kroah-Hartman wrote:
>>>>>>>> On Fri, Nov 24, 2023 at 03:21:40PM +0000, Mark Brown wrote:
>>>>>>>
>>>>>>>>> This came out of some discussions about trying to handle emergency power
>>>>>>>>> failure notifications.
>>>>>>>
>>>>>>>> I'm sorry, but I don't know what that means.  Are you saying that the
>>>>>>>> kernel is now going to try to provide a hard guarantee that some devices
>>>>>>>> are going to be shut down in X number of seconds when asked?  If so, why
>>>>>>>> not do this in userspace?
>>>>>>>
>>>>>>> No, it was initially (or when I initially saw it anyway) handling of
>>>>>>> notifications from regulators that they're in trouble and we have some
>>>>>>> small amount of time to do anything we might want to do about it before
>>>>>>> we expire.
>>>>>>
>>>>>> So we are going to guarantee a "time" in which we are going to do
>>>>>> something?  Again, if that's required, why not do it in userspace using
>>>>>> a RT kernel?
>>>>>
>>>>> For the HW in question I have only 100ms time before power loss. By
>>>>> doing it over use space some we will have even less time to react.
>>>>
>>>> Why can't userspace react that fast?  Why will the kernel be somehow
>>>> faster?  Speed should be the same, just get the "power is cut" signal
>>>> and have userspace flush and unmount the disk before power is gone.  Why
>>>> can the kernel do this any differently?
>>>>
>>>>> In fact, this is not a new requirement. It exist on different flavors of
>>>>> automotive Linux for about 10 years. Linux in cars should be able to
>>>>> handle voltage drops for example on ignition and so on. The only new thing is
>>>>> the attempt to mainline it.
>>>>
>>>> But your patch is not guaranteeing anything, it's just doing a "I want
>>>> this done before the other devices are handled", that's it.  There is no
>>>> chance that 100ms is going to be a requirement, or that some other
>>>> device type is not going to come along and demand to be ahead of your
>>>> device in the list.
>>>>
>>>> So you are going to have a constant fight among device types over the
>>>> years, and people complaining that the kernel is now somehow going to
>>>> guarantee that a device is shutdown in a set amount of time, which
>>>> again, the kernel can not guarantee here.
>>>>
>>>> This might work as a one-off for a specific hardware platform, which is
>>>> odd, but not anything you really should be adding for anyone else to use
>>>> here as your reasoning for it does not reflect what the code does.
>>>
>>> I see. Good point.
>>>
>>> In my case umount is not needed, there is not enough time to write down
>>> the data. We should send a shutdown command to the eMMC ASAP.
>>
>> If you don't care about the data, why is a shutdown command to the
>> hardware needed?  What does that do that makes anything "safe" if your
>> data is lost.
> 
> It prevents HW damage. In a typical automotive under-voltage labor it is
> usually possible to reproduce X amount of bricked eMMCs or NANDs on Y
> amount of under-voltage cycles (I do not have exact numbers right now).
> Even if the numbers not so high in the labor tests (sometimes something
> like one bricked device in a month of tests), the field returns are
> significant enough to care about software solution for this problem.
> 
> Same problem was seen not only in automotive devices, but also in
> industrial or agricultural. With other words, it is important enough to bring
> some kind of solution mainline.
> 

IMO that is a serious problem with the used storage / eMMC in that case and it
is not suitable for industrial/automotive uses?
Any industrial/automotive-suitable storage device should detect under-voltage and
just treat it as a power-down/loss, and while that isn't nice for the storage device,
it really shouldn't be able to brick a device (within <1M cycles anyway).
What does the storage module vendor say about this?

BR,
Christian